how can i get my agi last year 2019 if i don't have copy and also signed in 2020 with different username

You need your 2019 AGI to efile 2020. The AGI is on 2019 1040 line 8b. If you filed a Joint return you use the same AGI for each spouse.

How to find the AGI.
https://ttlc.intuit.com/community/agi/help/how-do-i-find-last-year-s-agi/01/25947

If you didn't file 2019 or filed it later in the year then try entering 0 for the AGI. If that doesn't work you will have to print and mail your return.

Or request a transcript from the IRS
https://www.irs.gov/individuals/get-transcript